The present invention relates to an ethylene copolymer, and more particularly to an in-situ prepared ethylene copolymer resin which has unique melt elastic properties when the resin is in its reactor-made or pelletized forms. The melt elastic properties observed by the ethylene copolymer resin of the present invention are not found in ethylene copolymers known heretofore, and importantly provide enhanced-impact strength properties to films that are produced therefrom.
The present invention is also directed to a polymerization catalyst.
The successful development of linear low density polyethylene (LLDPE) has forever changed the character of the polyethylene industry. For over fifty years, low density polyethylene (LDPE) was produced at pressures ranging up to 345 MPa (50,000 psi) and temperatures of about 300xc2x0 C. Technology was then developed in subsequent years which was capable of operating at less than 2 MPa (300 psi) and near about 100xc2x0 C., This technologic development has rapidly established itself as a low cost route to producing LLDPE.
LLDPE, which is typically made using a transition metal catalyst rather than a free-radical catalyst, as required for LDPE, is characterized by linear molecules having no long-chain branching; short-chain branching is instead present and is the primary determinant of resin density. The density of commercially available LLDPE typically ranges from 0.915-0.940 g/cm3. Moreover, commercially available LLDPEs. generally exhibit a crystallinity of from about 25-60 vol. %, and a melt index which can range from 0.01 g/10 min. to several hundred g/10 min.
Many commercial LLDPEs are available which contain one or more comonomers such as propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and mixtures thereof. The specific selection of a comonomer for LLDPE is based primarily on process compatibility, cost and product design.
In today""s polyethylene industry, LLDPEs are used in a wide variety of applications including film forming, injection molding, rotomolding, and wire and cable fabrication. A principal area for LLDPE copolymers is in film forming applications since such copolymers typically exhibit high dart impact, high Elmendorf tear, high tensile strength and high elongation, in both the machine direction (MD) and the transverse direction (TD), compared with counterpart LDPE resins.
Examples of previous developmental trends in this field include U.S. Pat. Nos. 5,26.0,245; 5,336,652; and 5,561,091, all to Mink et al., which disclose LLDPE films that exhibit the above properties made from polymerizing ethylene and at least one comonomer in the presence of a a polymerization cocatalyst and vastly distinct transition metal catalysts. Specifically, in the ""245 patent the transition metal catalyst is formed by treating silica having reactive OH groups with a dialkylmagnesium compound it a solvent; adding to said solvent a carbonyl-containing compound and then treating with a transition metal compound.
In the ""652 patent, the transition metal catalyst is prepared by treating a support having a reduced surface OH content with an organomagnesium compound; treating the product with a silane compound having the formula Rx1SiRy2 wherein R1 is Rwxe2x80x94O where Rw is hydrocarbyl containing 1 to 10 carbon atoms; R2 is halogen, hydrocarbyl having 1 to 10 carbon atoms or hydrogen; x is 1, 2, 3 or 4 and y is 0, 1, 2 or 3 with the proviso that x+y=4, and a transition metal compound. In this reference, reduction of surface OH content of the silica is effectuated by heating or by treatment with an aluminum compound.
The transition metal catalyst employed in the ""091 patent is one that is obtained by contacting silica having reactive OH groups with a dialkylmagnesium compound in a solvent; adding a mixture of an alcohol and SiCl4 thereto with subsequent treatment with a transition metal catalyst.
U.S. Pat. No. 4,335,016 to Dombro provides a supported olefin polymerization catalyst which is prepared by (1) forming a mixture of a calcined, finely divided porous support material and an alkyl magnesium compound; (2) heating the mixture for a time and at a temperature sufficient to react the support and the alkyl magnesium compound, (3) reacting, by heating, the product of (2) with a hydrocarbylhydrocarbyloxysilane compound; (4) reacting, by heating, the product of (3) with a titanium compound that contains a halide; or (5) reacting the product of (2) with the reaction product of a hydrocarbylhydrocarbyloxysilane compound and a titanium compound that contains a halide; and (6) activating the catalyst product of (4) or (5) with a cocatalyst comprising hydrogen or an alkyl lithium, alkyl magnesium, alkyl aluminum, alkyl aluminum halide or alkyl zinc.
Crotty et al. xe2x80x9cProperties of Superior Strength Hexene Film Resinsxe2x80x9d, Antec, 193, pp. 1210 describes the properties of superior strength hexene copolymer resins that are prepared by the Unipol process. These resins reportedly yield films with exceptional strength properties (impact and tear strength) that are significantly higher than the standard hexene products and even higher than achieved with commercially available octene copolymers. At the same time, the resins show little or no difference in processability from standard LLDPE.
The actual physical structures of polymers and abundant changes to same under various conditions is difficult to measure precisely and is commonly done indirectly. Rheology is often used in this regard, being especially suited to study the physical changes of polymers. Specifically, rheology deals with the deformation and flow of a polymer. Data so generated is used to provide information regarding the processability and even structural characterizations of the polymer.
One Theological method that is typically used is conventional, high shear modification wherein disentanglement of the polymer or copolymer chains occur. If a polymer or copolymer melt is sheared mechanically, the melt may be processed in a less elastic state or possibly less viscous state than the initial resin. Effects of shear modification are typically manifested by changes in die swell, die entrance pressure losses, normal stresses and flow defects such as sharkskin surfaces and melt fracture.
Although shear modification has been observed in LDPE, wherein disentanglement of the long chain branching of the polymer can readily occur, there was contention as to whether LLDPE could be shear modified. The question was answered in an article by Teh, et al. entitled xe2x80x9cShear Modification of Linear Low Density Polyethylenexe2x80x9d, Plastics and Rubber Processing and Applications, Vol. 4, No. 2, pg. 157 (1984). In this article, LLDPE was shear modified by preshearing the LLDPE resin under high shear conditions ( greater than 3.9 secxe2x88x921) in an extruder. This study indicated that shear modification of the LLDPE polymer causes disentanglement to occur in the extruder, and that the relatively, disentangled polymer can be restored to a more highly elastic, entangled state by subjecting the melt to annealing or dissolving the shear modified polymer in a solvent.
Another rheological technique employed in the prior art to determine the physical characteristics of a polymer is to measure the polydispersity or melt elasticity, ER, of the polymer melt. This technique is described in an article by R. Shroff, et al. entitled xe2x80x9cNew Measures of Polydispersity from Rheological Data on Polymer Meltsxe2x80x9d, J. Applied Polymer Science, Vol. 57, pp. 1605-1626 (1995).
Using this Theological technique (ER calculation), prior art ethylene copolymer resins, such as described in Teh, et al., exhibit conventional melt elastic behavior in both the unsheared pelletized and sheared pelletized states. In the unsheared state, the ER values of prior art ethylene copolymers remain substantially unchanged in going from the powder to pellet form. Moreover, no change in ER is observed in dissolving the pellet in an organic solvent.
As to the shear modified forms, prior art polymers exhibit a decrease in melt elasticity upon shear modification of the pelletized form. This signifies that the entanglement density of the polymer decreases. Upon dissolution of the shear modified form in an organic solvent, an increase in melt elasticity is observed with prior art ethylene copolymers. This increase in melt elasticity signifies a reversion of the polymer back to an entangled state.
In prior art ethylene copolymers, no polymeric networks, i.e. systems of interconnected macromolecular chains, are present. This is verified by the above melt elastic behavior of prior art ethlylene copolymers. As is known to those skilled in the art, the presence of network structures in polymers often provides polymers having improved properties. It is emphasized that while network structures are common in styrene-butadiene-styrene (SBS) block copolymersxe2x80x94See F. Morrison, et al., xe2x80x9cFlow-Induced Structure and Rheology of a Triblock Copolymerxe2x80x9d, J. Appl. Polymer Sci., Vol. 33, 1585-1600 (1987)xe2x80x94they are not known in LLDPE resins, until the advent of the present invention.
The present invention provides an ethylene copolymer that exhibits unique melt elastic properties that are not present in ethylene copolymers known heretofore. The unique melt elastic properties that are exhibited by the inventive ethylene copolymer are believed to be manifested by the presence of a network structure in the copolymer resin. While not being bound by any theory, it is hypothesized that the network structure in the present ethylene copolymer is formed at least in part of a rubber phase believed present in the copolymer which serves to interconnect the hard and soft phases of the ethylene copolymer.
The presence of a network structure in the ethylene copolymer resin of the present invention is verified by the fact that the copolymer resin exhibits a reactor-made-to-pellet ER increase which is reversible, i.e. reduced, upon rheometric low shear modification. The term xe2x80x9cERxe2x80x9d is used herein to measure the elasticity or the polydispersity of the ethylene copolymer which is derived from rheological data on polymer melts, See the article to Shroff, et al. supra. The term xe2x80x9creactor-madexe2x80x9d is used herein to denote powder, slurry or solution forms of the polymer resin which are formed in a polymerization vessel prior to melt processing.
In addition to exhibiting, the above melt elastic behavior, the pelletized form of the ethylene copolymer of the present invention exhibits a decrease in melt elasticity when dissolved in an organic solvent such as xylene. The solution dissolution ER value is nearly the same as that of the original reactor-made material.
Specifically, the ethylene copolymer resin of the present invention comprises ethylene, as the major component, and at least one C4-8 comonomer with the proviso that the resin, when in pelletized form, has a reduction in melt elasticity (ER) of 10% or more, to a final ER value of 1.0 or less upon rheometric low shear modification or solution dissolution. A 10-30% reduction in ER of the pelletized form of the inventive copolymer resin upon rheometric low shear modification or solution dissolution is typically observed. Moreover, the ethylene copolymer resin of the present invention, when in reactor-made form, exhibits a partially reversible increase of 10% or more in said ER when pelletizing the same.
The term xe2x80x9crheometric low shear modificationxe2x80x9d is used in the present invention to indicate that the modification occurs in a rheometer that is capable of operating at shear rates of less than 1.0 sect for a time period of from about 10 to about 60 minutes. This term is thus distinguishable from high shear modification, as disclosed in Teh, et al., supra, wherein the modification is typically carried out in an extruder, prior to being introduced into a rheometer, at shear rates of 3.9 secxe2x88x921 and higher.
The term xe2x80x9csolution dissolutionxe2x80x9d is used herein to indicate that the pelletized form of the ethylene copolymer resin can be dissolved in an organic solvent such as xylene. The importance of this technique is that it allows a means for estimating the ER value of the original reactor-made material if the same is not available.
In addition to exhibiting unique melt elastic properties, the ethylene copolymer resin of the present invention is further characterized as having a base polymer density of about 0.930 g/cm3 or less, a melt index of from about 0.01 g/10 min or greater and a rubber content of about 15 vol. % or greater. Moreover, the rubber phase of the ethylene copolymer resin of the present invention contains from about 35 to about 65 alkyl branches per 1000 total carbon atoms.
Another aspect of the present invention relates to a high-impact strength film that can be produced from the ethylene copolymer resin of the present invention. The term xe2x80x9chigh-impact strengthxe2x80x9d is used herein to denote an impact strength, as measured using a free-falling dart, of at least about 30.0 g/mil or higher.
Another aspect of the present invention relates to a polymerization catalyst which, among other things, is capable of producing ethylene copolymers having the unique melt elastic properties mentioned above. In one embodiment of the present invention, the ethylene polymerization catalyst is obtained by:
(a) contacting a support material with an organosilicon compound to effectuate reduction of surface hydroxyl groups present on said support material;
(b) contacting the modified support material with a dialkylmagnesium compound or complex;
(c) contacting the product of (b) with an alcohol or a hydrocarbyloxyhydrocarbylsilane; and
(d) contacting the product of (c) with a transition metal compound.
In another embodiment of the present invention, the polymerization catalyst is obtained by:
(a) contacting a support material with an organosilicon compound to effectuate reduction of surface hydroxyl groups present on said support material;
(b) contacting the modified support material with a dialkylmagnesium compound or complex;
(c) contacting the product of (b) with a transition metal compound; and
(d) contacting the product of (c) with an alcohol or a hydrocarbyloxyhydrocarbylsilane.
In yet another embodiment, the catalyst of the present invention is obtained by:
(a) contacting a support material with an organosilicon compound to effectuate reduction of surface hydroxyl groups present on said support material;
(b) contacting the modified support material with an alcohol or a hydrocarbyloxyhydrocarbylsilane;
(c) contacting the product of (b) with a dialkylmagnesium compound or complex; and
(d) contacting the product of (c) with a transition metal compound.
It is emphasized that in the embodiments wherein an alcohol is employed in preparing the polymerization catalyst, a hydrocarbyl alkoxysilane cocatalyst modifier such as diisopropyldimethoxysilane (DIPS) is required to be used.
A still further aspect of the present invention relates to an ethylene polymerization process wherein ethylene and at least one C4-8 comonomer are copolymerized in the presence of one of the above-mentioned ethylene polymerization catalysts, a suitable cocatalyst capable of activating the ethylene polymerization catalyst and, optionally, a cocatalyst modifier. This polymerization process results in the production of the inventive ethylene copolymer resin having the unique melt elastic properties described hereinabove.